The interaction between prior antimicrobial drug exposure and resistance in human Salmonella infections

The use of antimicrobial drugs for food animals selects for resistant non-typhoid Salmonella strains, but human consumption of antimicrobial drugs may also increase the risk of subsequent infection. The aim of this study was to determine the risk of salmonellosis attributable to human consumption of antimicrobial drugs in a case-control study of 22 602 laboratory-confirmed Salmonella infections, diagnosed in Denmark between 1997 and 2005. A population registry-based case-control study, using several Danish databases: the National Prescription Database; the National Registry for Enteric Pathogens; the Civil Registry System; and the Integrated Database on Labour Market Research. Exposure to trimethoprim, sulphonamides, broad-spectrum penicillins, tetracyclines and fluoroquinolones, during the year prior to diagnosis, was associated with an increased risk of non-typhoid Salmonella infection. Overall, the highest risk was associated with the prior use of fluoroquinolones. This risk increased as the time window of exposure approached the infection date. Previous use of fluoroquinolones was associated with an odds ratio (OR) of 4.55 [95% confidence interval (CI): 3.78-5.47] for Salmonella serotypes other than Salmonella Typhimurium or Salmonella Enteritidis, an OR of 2.21 (95% CI: 1.70-2.86) for Salmonella Typhimurium and an OR of 2.07 (95% CI: 1.76-2.42) for Salmonella Enteritidis. In particular for fluoroquinolones, there was an interaction between the pathogen resistance pattern and a history of antibiotic drug use. The increasing use of antibiotics, particularly fluoroquinolones, is likely to result in increased incidence of foodborne infections with drug-resistant Salmonella.